Everything You Need To Know To Find The Best Tailings Pipeline

Brumadinho is a municipality in the Brazilian state of Minas Gerais, in the country’s mineral-rich southwest. In ,  “Dam I” at an iron ore mine about five miles outside the city collapsed. Dam I was a tailings dam, storing bi-products of mining. Tailings are mine waste, often a toxic slurry of dirt, water, and rocks, with metal and chemicals mixed in. With no place else to go, it’s expediently stored in earthen retaining structures called tailings dams situated near the mine. The dams themselves may be constructed from tailings.

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When Dam I failed, 12 million cubic meters (3.2 billion gallons) of tailings rushed through a nearby valley for 10 brutal kilometers (6.2 miles). The official death toll was 270, including 11 people who have yet to be recovered. Agriculture suffered. Pollution from the collapse reached water supplies in 21 municipalities up to 120 km (75 miles) from the collapse site. You can still see the scar, and Brumaninho is now defined by the disaster it suffered. And this was not even the first tailings dam failure in the state of Minas Gerais, coming just four years after the human and environmental catastrophe of a tailings dam collapse in Mariana. 

The List of Past and Potential Failures Is Long

There have been over 200 documented tailings dam (1) failures in the last hundred years, and many are never reported. A paper published in Environmental Sciences estimates the failure rate of tailings dams at 1.2%, “more than two orders of magnitude higher than the failure rate of conventional water retention dams, reported to be 0.01%.”  Failures occur approximately every eight months (2). 

There is no official registry of tailings dams, but estimates generally put the total number at about 18,000 worldwide, with approximately 3,500 active. 

It should be noted that Dam 1 had been inactive for seven years prior to failure. Tailings dam integrity monitoring can be an expensive proposition for a mine that no longer produces; one of the major causes of tailings dams failures is inattention to dams where operations have ceased.

Why Tailings Dams Fail 

Several reasons for tailings dams failures have already been mentioned. Using mine waste to build dams that contain mine waste is about cost, not good engineering. Whether the dam is operating or inactive, proper maintenance is expensive with no profit to show. Climate change-driven precipitation, seen in more frequent, severe, and widespread flooding events, puts additional strain on these structures.  The Syncrude Tailings Dam in Canada is the world’s tallest, reaching 88 meters at points, equal to a 26-story building, and it can hold 540 million cubic meters, almost 6,500 times the volume released by Dam I. Taller dams, it should noted, are more likely to fail. 

The failure at Dam 1 was caused by static liquefaction, a sudden loss of strength when loose material such as sand can no longer drain. Other causes include: overtopping, where the content level and wind setup exceed the crest of the dam; sequential raising of the dam (making it taller at time intervals to accommodate more volume) instead of designing and building a finished product; failure of the surrounding slopes; seismic events; and loose regulations, poor enforcement, or both. 

Seepage, or water slowly permeating through the dam and its foundation, can also lead to failure if left uncontrolled. All earthen dams, (most tailings dams are earthen dams, made of earthen materials) feature some amount of seepage. If velocity and quantity reach the point of eroding soil from the foundation or embankment, the dam can fail. Seepage can also saturate and weaken the slopes, again leading to failure.  

And time. Time is a problem. 

There is no end date on the required time a tailings dam must last. “More than 10,000 years” (twice the length of recorded human history) was suggested in a study published by the Center for Science in Public Participation (3). For reference, Dam I was 47 years old.  

Tailings Dam Prevention and the Role of Monitoring 

There are a number of steps that could be taken to reduce the risk of failure, including better regulations, better enforcement, and more money allocated to maintenance. But there is little incentive. Mine operators may be punished for the dams that fail, but are rarely applauded or rewarded for those that don’t. Failure may be seen as the cost of doing business. 

Another solution is better tailings dams monitoring. In fact, the International Council on Mining & Metals, in its Global Standards on Tailings Management, places a very high priority on better and more extensive tailings dam monitoring. 

A Significant Advance in Tailings Dam Monitoring

Many of the techniques used to monitor tailing dams are limited in their range, effectiveness, or both. Traditional ground penetrating radar can be used but is less effective on dynamic structures like tailings dams than on solid structures.  Laser scanners have been used to detect movements in a dam wall, but the hardware investment is prohibitive and the technology works best only in specific situations. Seismic sensors have their own limitations. Other radars, such as InSAR, show movement, but not the underlying cause.

ASTERRA EarthWorks Monitors Tailings Dams from Space

EarthWorks uses satellite-based SAR (synthetic aperture radar) to bring an entirely new capability to tailings dam monitoring: the ability to locate and monitor soil moisture seeping through or under tailings dams. 

Carried on a band of the spectrum called the L-band, SAR can discern soil moisture several feet below the ground. And it can survey even the largest tailings dams at once. (The Syncrude dam has an 18-km (11-mile) circumference.) Repeated satellite passes will discern changes over time that precede larger problems so preventive measures can be taken.

As we mentioned, seepage can lead to tailings dam failure through several different mechanisms. An affordable, accessible technology for ongoing, wide-area soil moisture monitoring of tailings dams has always been a necessity. Now, with ASTERRA EarthWorks, it’s also a practical reality. 

Interested in learning more about the role of EarthWorks in monitoring tailings dams from space? We invite you to browse our products and services or get in touch with us today!  

No two slurries are the same. That makes tailings transport a challenge, even for the most seasoned pipeline engineers. Fully understanding what you’re transporting could mean the difference between a straightforward pipeline project or one plagued with problems.

Dr Lachlan Graham is a research engineer in fluid dynamics at CSIRO. He says slurry rheology—the physics of the flow of matter—is often overlooked when designing pipelines.

“Slurry rheology range is a key influencer in pipeline design.  At this stage in the process, you need to know the rheological properties of the slurry you’re working with,” he says.

“Otherwise, you could be in big trouble later on.”

Rheology testing is the most accurate way to understand how slurry behaves, but Lachlan says you can also apply basic principles to identify potential issues before they occur.

And that’s why we created this article. It gives you an introduction to the basic principles of rheology so you can make better design decisions on your next slurry pipeline.

Non-settling and settling slurries

A non-settling slurry is a uniform mixture of solids and liquids—the particles stay in suspension and don’t settle to the bottom. These particles are typically small (less than 60-100 microns).

Particles in a settling slurry are only in suspension when there’s agitation due to turbulence. Once pumping stops, the particles settle to the bottom of the pipe, forming a low-viscosity carrier fluid.

In the diagram below, you can see how different slurry types behave based on particle size and transport speed.

“If the particle concentration is the same at the top as at the bottom of the pipe, you tend to get reasonably even wear all around the pipe,” explains Lachlan.

“If you drop the velocity a little bit, you’ll start to see stratification. In that situation, you tend to get wear towards the bottom of the pipe.

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With a static, non-moving bed, you’ll see very little wear on the base because nothing’s moving. But you’ll get a lot of wear outside of that bed.”

That determines the type of piping you’ll need. With a non-settling slurry, you may get away with using unlined steel or HDPE piping.

Settling slurries cause higher, uneven wear in pipelines. There are two reasons for this:

1. Often pumping speed is increased to reduce settling. That increases turbulence in the pipe and erosive wear.
2. The settled bed causes sliding bed abrasion on the bottom of the pipe. In this case, it’s best to use high wear lining like polyurethane.

Newtonian and Non-Newtonian slurries

Another way of classifying slurries is as Newtonian and non-Newtonian fluids.

With Newtonian fluids, the viscosity remains the same, irrespective of the shear rate at a constant temperature. Some examples of Newtonian fluids are water, alcohol, or petrol.

Non-Newtonian fluids behave differently. With these, the viscosity of the slurry changes when shear stress is applied.

There are different types of non-Newtonian fluids. Tailings are often modelled as Bingham Plastic together with a coarse particle fraction. This is included in the pipe design models if there is a significant concentration of particles larger than 75 microns. These slurries won’t flow in a pipe until a minimum stress (such as pumping) is applied to overcome their yield stress.

The graph below shows how Newtonian and non-Newtonian fluids behave when shear stress is applied.

Whether a slurry behaves as a Newtonian or non-Newtonian fluid changes the pumping and transport regime. For example, higher viscous fluids like paste backfill may mean using a positive displacement pump rather than a centrifugal pump. With a more dilute slurry, you’ll be able to use a centrifugal pump but you’ll have to look at liner options to cope with turbulence of settling slurry particles.

That leads us to friction loss. Some liner technologies – like rubber-lined steel – have high friction loss. That’s a big deal for long-distance slurry pipelines because it can dramatically increase your pumping requirements.

Newer liners like polyurethane have very low friction loss, which reduces the head loss caused by frictional resistance on the pipe wall. That means you can run with lower pump power, potentially saving capital costs if you can use smaller or fewer pumps, and also reducing your energy operating costs.

Viscosity

Viscosity is a fluid’s resistance to flow. A fluid with a high viscosity (such as honey) is more resistant to flow versus a fluid with low viscosity (water).

The viscosity of different tailings varies. Thickened tailings and paste have a high viscosity, whereas iron ore tailings for example are usually more dilute.

Lachlan says that, generally, the lower the viscosity of the slurry, the higher the physical wear in a pipeline.

“If your carrier fluid is more viscous, towards the ‘toothpaste’ end of the spectrum, the fluid tends to carry the coarse particles around obstacles in the flow,” he says.

“Particles are less likely to impact upon the pipe surface as the viscosity goes up. As the viscosity increases, there’s less erosive wear.”

On the other hand, a dilute slurry, such as iron ore tailings, has a lower viscosity and therefore needs to be pumped at higher velocity to reduce settling.

Pumping at higher velocity has a downside though. It increases the velocity of solid particles hitting the walls of the pipe which in turn increases wear on the pipe walls. This means that bare carbon steel or HDPE lined pipe is typically not suitable for high velocity slurries.

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Your goal with a new slurry pipeline should be to design it right the first time, so it lasts the life of the project. The alternative is failure followed by downtime and expensive maintenance.

That’s why the true test of your pipeline project’s success comes in the months and years after. If you haven’t factored in the slurry properties, it could end up being a maintenance nightmare.

Lachlan says it pays to get expert guidance early on because slurries can vary greatly from the ore type to the type of processing in the plant.

“It’s likely that the operating conditions will change throughout the process life of a plant. An example of this is if the ore body or process conditions in the plant change.

“So, you need to design for a possible range of conditions and know the range of rheology as best you can.”

Conducting rheology testing, or slurry loop testing together with pipe design models will give you the most accurate data about how your slurry will behave, so you can iron out issues before they occur.

It pays to know what you’re pumping

Slurry rheology has a big impact on pipeline wear and performance.

Having a good understanding of this will help you design a better pipeline that will last the length of the project.

It pays to do in-depth analysis of the settling properties, Newtonian vs non-Newtonian behaviour, and viscosity of the slurry that your pipeline will transport.

It could be the difference between a pipeline that’s seen as a liability to the operation, or one that’s a valuable asset.